NMR structure of the sea urchin (Strongylocentrotus purpuratus) metallothionein MTA

Homo- and Heteronuclear Group 12 Metallothionein Type B Cluster Analogs: Synthesis, Structure, 1H NMR and ESI-MS

Metallothioneins (MTs) are a ubiquitous class of small cysteine-rich metal-binding proteins involved in metal homeostasis and detoxification with highly versatile metal binding properties. Despite the long-standing association of MT with M₃S₃ and M₄S₅ metal clusters, synthetic complexes with these core architectures are exceptionally rare. Here, we demonstrate an approach to synthesizing and characterizing aggregates of group 12 metal ions with monocyclic M₃S₃ cores in acetonitrile solution without the protection of a protein. Multidentate monothiol ligand N,N-bis(2-pyridylmethyl)-2-aminoethanethiol (L1H) provided [Cd₃(L1)₃](ClO₄)₃ (1), the first structurally characterized nonproteinaceous aggregate with a metallothionein-like monocyclic Cd₃S₃ core. In addition, [Zn₃(L1)₃](ClO₄)₃·4CH₃CN (2·4CH₃CN) was characterized by X-ray crystallography. The complex cations of 1 and 2 had comparable structures despite being nonisomorphic. Variable temperature and concentration ¹H NMR were used to investigate aggregation equilibria of 1, 2, and a precipitate with composition "Hg(L1)(ClO₄)" (3). Cryogenic ¹H NMR studies of 3 revealed a J(¹⁹⁹Hg¹H) coupling constant pattern consistent with an aggregate possessing a cyclic core. ESI-MS was used for gas-phase characterization of 1-3, as well as mixed-metal [M₂M'(L1)₃(ClO₄)₂]⁺ ions prepared in situ by pairwise acetonitrile solution combinations of the group 12 complexes of L1. Access to synthetic variants of metallothionein-like group 12 aggregates provides an additional approach to understanding their behavior.

Tunicates Illuminate the Enigmatic Evolution of Chordate Metallothioneins by Gene Gains and Losses, Independent Modular Expansions, and Functional Convergences

To investigate novel patterns and processes of protein evolution, we have focused in the metallothioneins (MTs), a singular group of metal-binding, cysteine-rich proteins that, due to their high degree of sequence diversity, still represents a "black hole" in Evolutionary Biology. We have identified and analyzed more than 160 new MTs in nonvertebrate chordates (especially in 37 species of ascidians, 4 thaliaceans, and 3 appendicularians) showing that prototypic tunicate MTs are mono-modular proteins with a pervasive preference for cadmium ions, whereas vertebrate and cephalochordate MTs are bimodular proteins with diverse metal preferences. These structural and functional differences imply a complex evolutionary history of chordate MTs-including de novo emergence of genes and domains, processes of convergent evolution, events of gene gains and losses, and recurrent amplifications of functional domains-that would stand for an unprecedented case in the field of protein evolution.

Modular Evolution and Population Variability of Oikopleura dioica Metallothioneins

Chordate Oikopleura dioica probably is the fastest evolving metazoan reported so far, and thereby, a suitable system in which to explore the limits of evolutionary processes. For this reason, and in order to gain new insights on the evolution of protein modularity, we have investigated the organization, function and evolution of multi-modular metallothionein (MT) proteins in O. dioica. MTs are a heterogeneous group of modular proteins defined by their cysteine (C)-rich domains, which confer the capacity of coordinating different transition metal ions. O. dioica has two MTs, a bi-modular OdiMT1 consisting of two domains (t-12C and 12C), and a multi-modular OdiMT2 with six t-12C/12C repeats. By means of mass spectrometry and spectroscopy of metal-protein complexes, we have shown that the 12C domain is able to autonomously bind four divalent metal ions, although the t-12C/12C pair –as it is found in OdiMT1– is the optimized unit for divalent metal binding. We have also shown a direct relationship between the number of the t-12C/12C repeats and the metal-binding capacity of the MTs, which means a stepwise mode of functional and structural evolution for OdiMT2. Finally, after analyzing four different O. dioica populations worldwide distributed, we have detected several OdiMT2 variants with changes in their number of t-12C/12C domain repeats. This finding reveals that the number of repeats fluctuates between current O. dioica populations, which provides a new perspective on the evolution of domain repeat proteins.

Modularity in Protein Evolution: Modular Organization and De Novo Domain Evolution in Mollusk Metallothioneins

Metallothioneins (MTs) are proteins devoted to the control of metal homeostasis and detoxification, and therefore, MTs have been crucial for the adaptation of the living beings to variable situations of metal bioavailability. The evolution of MTs is, however, not yet fully understood, and to provide new insights into it, we have investigated the MTs in the diverse classes of Mollusks. We have shown that most molluskan MTs are bimodular proteins that combine six domains—α, β1, β2, β3, γ, and δ—in a lineage-specific manner. We have functionally characterized the Neritimorpha β₃β₁ and the Patellogastropoda γβ₁ MTs, demonstrating the metal-binding capacity of the new γ domain. Our results have revealed a modular organization of mollusk MT, whose evolution has been impacted by duplication, loss, and de novo emergence of domains. MTs represent a paradigmatic example of modular evolution probably driven by the structural and functional requirements of metal binding.

Characterisation of the metallothionein gene in the Sydney rock oyster and its expression upon metal exposure in oysters with different prior metal exposure histories

The metal-binding protein metallothionein (MT) is widely used as a biomarker of metal contamination. In this study, we cloned a MT gene (sgMT) from the Sydney rock oyster Saccostrea glomerata. The gene encodes a MT-I protein with a classical αβ domain structure and is expressed as two transcripts resulting from alternative polyadenylation. The gene promoter contains two putative metal-responsive elements (MREs) which are known to be required for metal-inducible transcription. A specific and efficient qPCR assay was developed to quantify sgMT mRNA expression. Further, we assessed whether prior metal exposure history influences sgMT mRNA expression upon subsequent metal exposure. Oysters with varying prior metal exposure histories (contaminated and reference) were exposed to Cu, Cd and Zn. Expression of sgMT generally increased with metal dose, and oysters with an elevated past metal exposure history exhibited higher sgMT expression under Cd and Zn stress, representing a potential acclimatory response to prior metal exposure.

Functional comparision between truncated MTT1 and truncated MTT2 from Tetrahyemna thermophila

Metallothioneins (MTs) are low-molecular-weight proteins with high Cys content and high metal-chelating ability. CdMT and CuMT subfamilies present different characteristics in Tetrahymena. To explore the effect of the cysteine arrangement and sequence length of MTs for binding different metal ions, MTT1, truncated MTT1 (TM1), MTT2, and truncated MTT2 (TM2) were expressed in E. coli. The half-maximal inhibiting concentrations (IC50) of Cd2+ and Cu+ for the recombinant strains were different. Furthermore, E. coli cells expressing MTT1 and TM1 exhibited higher accumulating ability for Cd2+ than cells expressing MTT2 and TM2. However, the opposite is true for Cu+. The binding ability of the different recombinant proteins to Cd2+ and Cu+ were also different. MTT1 and truncated mutant TM1 were the preference for Cd2+, whereas MTT2 and truncated mutant TM2 were the preference for Cu+ coordination. These results showed that metal ion tolerance and accumulation ability not only depended on cysteine arrangement pattern but also on sequence length of MT in Tetrahymena.

Engineering Mycorrhizal Symbioses to Alter Plant Metabolism and Improve Crop Health

Creating sustainable bioeconomies for the 21st century relies on optimizing the use of biological resources to improve agricultural productivity and create new products. Arbuscular mycorrhizae (phylum Glomeromycota) form symbiotic relationships with over 80% of vascular plants. In return for carbon, these fungi improve plant health and tolerance to environmental stress. This symbiosis is over 400 million years old and there are currently over 200 known arbuscular mycorrhizae, with dozens of new species described annually. Metagenomic sequencing of native soil communities, from species-rich meadows to mangroves, suggests biologically diverse habitats support a variety of mycorrhizal species with potential agricultural, medical, and biotechnological applications. This review looks at the effect of mycorrhizae on plant metabolism and how we can harness this symbiosis to improve crop health. I will first describe the mechanisms that underlie this symbiosis and what physiological, metabolic, and environmental factors trigger these plant-fungal relationships. These include mycorrhizal manipulation of host genetic expression, host mitochondrial and plastid proliferation, and increased production of terpenoids and jasmonic acid by the host plant. I will then discuss the effects of mycorrhizae on plant root and foliar secondary metabolism. I subsequently outline how mycorrhizae induce three key benefits in crops: defense against pathogen and herbivore attack, drought resistance, and heavy metal tolerance. I conclude with an overview of current efforts to harness mycorrhizal diversity to improve crop health through customized inoculum. I argue future research should embrace synthetic biology to create mycorrhizal chasses with improved symbiotic abilities and potentially novel functions to improve plant health. As the effects of climate change and anthropogenic disturbance increase, the global diversity of arbuscular mycorrhizal fungi should be monitored and protected to ensure this important agricultural and biotechnological resource for the future.

Metallothionein Gene Family in the Sea Urchin Paracentrotus lividus: Gene Structure, Differential Expression and Phylogenetic Analysis

Metallothioneins (MT) are small and cysteine-rich proteins that bind metal ions such as zinc, copper, cadmium, and nickel. In order to shed some light on MT gene structure and evolution, we cloned seven Paracentrotus lividus MT genes, comparing them to Echinodermata and Chordata genes. Moreover, we performed a phylogenetic analysis of 32 MTs from different classes of echinoderms and 13 MTs from the most ancient chordates, highlighting the relationships between them. Since MTs have multiple roles in the cells, we performed RT-qPCR and in situ hybridization experiments to understand better MT functions in sea urchin embryos. Results showed that the expression of MTs is regulated throughout development in a cell type-specific manner and in response to various metals. The MT7 transcript is expressed in all tissues, especially in the stomach and in the intestine of the larva, but it is less metal-responsive. In contrast, MT8 is ectodermic and rises only at relatively high metal doses. MT5 and MT6 expression is highly stimulated by metals in the mesenchyme cells. Our results suggest that the P. lividus MT family originated after the speciation events by gene duplications, evolving developmental and environmental sub-functionalization.

Copper metallothioneins

Metallothioneins (MTs) are a class of low molecular weight and cysteine-rich metal binding proteins present in all the branches of the tree of life. MTs efficiently bind with high affinity several essential and toxic divalent and monovalent transition metals by forming characteristic polynuclear metal-thiolate clusters within their structure. MTs fulfil multiple biological functions related to their metal binding properties, with essential roles in both Zn(II) and Cu(I) homeostasis as well as metal detoxification. Depending on the organism considered, the primary sequence, and the specific physiological and metabolic status, Cu(I)-bound MT isoforms have been isolated, and their chemistry and biology characterized. Besides the recognized role in the biochemistry of divalent metals, it is becoming evident that unique biological functions in selectively controlling copper levels, its reactivity as well as copper-mediated biochemical processes have evolved in some members of the MT superfamily. Selected examples are reviewed to highlight the peculiar chemical properties and biological functions of copper MTs. © 2016 IUBMB Life, 69(4):236-245, 2017.

Metallothionein-3 Is a Component of a Multiprotein Complex in the Mouse Brain

Metallothlonein (MT)-3, originally called growth inhibitory factor (GIF), was initially identified through its ability to Inhibit the growth of neuronal cells in the presence of brain extract. MT-3 is the brain specific isoform of the MT family whose specific biological activity associates it with neurological disorders. Indeed, studies report that MT-3 is decreased by ~30% in brains of patients with Alzheimer disease (AD). Furthermore, many lines of evidence suggest that MT-3 engages in specific protein interactions. To address this, we conducted Immunoaffinity chromatography experiments using an immobilized anti-mouse MT-3 antibody. We identified five associated proteins from the pool of sixteen recovered using mass spectrometry and tandem mass spectrometry after in-gel trypsin digestion of bands from the affinity chromatography. The proteins identified were: heat shock protein 84 (HSP84), heat shock protein 70 (HSP70), dihydropyrimidinase-like protein-2 (DRP-2), creatine kinase (CK) and β-actin. Coimmunoprecipitation experiments, also conducted on whole mouse brain extract using the anti-mouse MT-3 antibody along with commercially available antibodies against HSP84 and CK, confirmed that these three proteins were in a single protein complex. Immunohistochemical experiments were then conducted on the perfused mouse brain that confirmed the in situ colocallzation of CK and MT-3 in the hippocampus region. These data provide new Insights into the involvement of MT-3 in a multiprotein complex, which will be used to understand the biological activity of MT-3 and its role in neurological disease.

Molecular characterization and expression of metallothionein from freshwater pearl mussel, Hyriopsis schlegelii

Two metallothionein genes (HsMT1 and HsMT2) were first identified and described from Hyriopsis schlegelii. The open reading frame of HsMT1 and HsMT2 were 216 and 222 bp, encoding a protein of 71 and 73 amino acid residues. The deduced amino acid sequences showed they contained parts of typical MT characteristics, apart from HsMT2 lacked Cys-Cys motifs. The phylogenetic tree showed HsMT1 shared a high similarity with that of other molluscs, but HsMT2 was split into a distinct group separated from known molluscan MTs. HsMT1 exhibited constitutive expression in all examined tissues and the highest expression occurred in hepatopancreas, however, nearly all HsMT2 was just detected in gonad. After Cd exposure, their mRNA levels presented similar expression patterns. The transgenic bacteria of HsMT1 showed higher tolerance than HsMT2 in Cd environment. It was implied that HsMT1 and HsMT2 were involved in metal response but HsMT2 might have other physiological functions.

Metal and metallothionein concentrations in Paracentrotus lividus from Amvrakikos gulf (Ionian Sea-Greece)

Concentrations of Cd, Cu, Cr, Ni, Zn, Fe and metallothioneins (MTs) were measured in the gonads of Paracentrotus lividus from Amvrakikos gulf (Ionian Sea, Greece). Three natural populations were selected; two of them, growing inside the gulf (Agios Thomas and Koronisia), presented higher density and smaller body size than the population living in a coastal area just outside the gulf (Mytikas). Metal and MT levels were not elevated, with the exception of Zn, showing high values related to the reproduction stage of the sea urchins. Significant geographical variations were recorded in the concentrations of Cu, Zn, Cd, Cr and MTs. The highest mean and maximum values of Cu, Zn and MTs were recorded in Agios Thomas while Cd and Cr were higher in Mytikas population. Copper, Zn, Fe and MT concentrations were negatively correlated to the sea urchin body size, while a positive concentration-size relationship was observed for Cd. Although all studied populations grow in a low metal level marine environment, urchins with smaller body size living in a food limited marine environment showed higher gonadosomatic index, metal concentrations and MT levels in their gonads (Agios Thomas and Koronisia) than larger specimens growing in a food unlimited area (Mytikas).

Challenging conventional wisdom: single domain metallothioneins

Metallation studies of human metallothioneins support the role of single metal-binding-domains as commonplace with the typical two-domain-cluster structure as exceptional.

Solution structure of the circular γ-domain analog from the wheat metallothionein E(c)-1

The first cyclic analog of a metallothionein (MT) was prepared and analyzed by UV and (magnetic) circular dichroism spectroscopy, ESI-MS as well as NMR spectroscopy. Results reveal that the evaluated cyclic γ-E_c-1 domain of the wheat MT E_c-1 retains its ability to coordinate two Zn(II) or Cd(II) ions and adopts a three-dimensional structure that is highly similar to the one of the linear wild-type form. However, the reduced flexibility of the protein backbone facilitates structure solution significantly and results in a certain stabilization of metal binding to the protein.

Tissue-specificity and phylogenetics of Pl-MT mRNA during Paracentrotus lividus embryogenesis

Metallothioneins (MTs) constitute a family of cysteine-rich, low molecular weight proteins, which generally provide protection against metal toxicity and oxidative stress counteracting the cell damage caused by essential and non-essential heavy metals. Equally important is the physiological role of MTs in the homeostasis of essential metals, which are involved in a wide variety of cellular processes. The aim of this work was to investigate the expression and the territorial localization of Paracentrotus lividus MT (Pl-MT) mRNA during sea urchin development by Quantitative Polymerase Chain Reaction (QPCR) and Whole Mount In Situ Hybridization (WMISH), as well as the phylogenetic comparison with selected MT homologs present in different phyla. We found that Pl-MT mRNA is accumulated in unfertilized eggs and constitutively expressed during development, with very low levels of maternal mRNA at cleavage stages, followed by a significant rise during gastrulation with a peak at the prism stage. Pl-MT mRNA was expressed in the vegetative plate at mesenchyme blastula, later restricted to the endoderm of gastrula embryos and finally to the gut of plutei. Indirect immunofluorescence (IF) using a specific antibody for the endoderm marker Endo1 demonstrated a co-localization with the Pl-MT transcripts in the midgut and hindgut after the intestine differentiation occurs and when larval feeding begins. Our results show for the first time the constitutive temporal and tissue-specific expression of MT in P. lividus embryos, providing new information for studies on the mechanisms controlling basal and induced MT gene expression. The analysis of the phylogenetic relationship of Pl-MT with homologs from different phyla, ranging from yeast to vertebrates, suggests the evolutionary process of these proteins, which could have been selected not only on the basis of their ability to bind metals but also by their tissue-specificity.

The sea urchin metallothionein system: Comparative evaluation of the SpMTA and SpMTB metal-binding preferences

Metallothioneins (MTs) constitute a superfamily of ubiquitous metal-binding proteins of low molecular weight and high Cys content. They are involved in metal homeostasis and detoxification, amongst other proposed biological functions. Two MT isoforms (SpMTA and SpMTB) have been reported in the echinoderm Strongylocentrotus purpuratus (sea urchin), both containing 20 Cys residues and presenting extremely similar sequences, although showing distinct tissular and ontogenic expression patterns. Although exhaustive information is available for the Cd(II)-SpMTA complex, this including the full resolution of its 3D structure, no data has been reported concerning either SpMTA Zn(II) and Cu(I) binding properties, or the characterization of SpMTB at protein level. In this work, both the SpMTA and SpMTB isoforms, as well as their separate α and β domains, have been recombinantly synthesized in the presence of Zn(II), Cd(II) or Cu(II), and the corresponding metal complexes have been analyzed using electrospray mass spectrometry, and CD, ICP-AES and UV-vis spectroscopies. The results clearly show a better performance of isoform A when binding Zn(II) and Cd(II), and of isoform B when coordinating Cu(I). Thus, our results confirm the differential metal binding preference of SpMTA and SpMTB, which, together with the reported induction pattern of the respective genes, highlights how also in Echinodermata the MT polymorphism may be linked to the evolution of different physiological roles.

Use of (113)Cd NMR to probe the native metal binding sites in metalloproteins: an overview

Our laboratories have actively published in this area for several years and the objective of this chapter is to present as comprehensive an overview as possible. Following a brief review of the basic principles associated with (113)Cd NMR methods, we will present the results from a thorough literature search for (113)Cd chemical shifts from metalloproteins. The updated (113)Cd chemical shift figure in this chapter will further illustrate the excellent correlation of the (113)Cd chemical shift with the nature of the coordinating ligands (N, O, S) and coordination number/geometry, reaffirming how this method can be used not only to identify the nature of the protein ligands in uncharacterized cases but also the dynamics at the metal binding site. Specific examples will be drawn from studies on alkaline phosphatase, Ca(2+) binding proteins, and metallothioneins.In the case of Escherichia coli alkaline phosphatase, a dimeric zinc metalloenzyme where a total of six metal ions (three per monomer) are involved directly or indirectly in providing the enzyme with maximal catalytic activity and structural stability, (113)Cd NMR, in conjunction with (13)C and (31)P NMR methods, were instrumental in separating out the function of each class of metal binding sites. Perhaps most importantly, these studies revealed the chemical basis for negative cooperativity that had been reported for this enzyme under metal deficient conditions. Also noteworthy was the fact that these NMR studies preceded the availability of the X-ray crystal structure.In the case of the calcium binding proteins, we will focus on two proteins: calbindin D(9k) and calmodulin. For calbindin D(9k) and its mutants, (113)Cd NMR has been useful both to follow actual changes in the metal binding sites and the cooperativity in the metal binding. Ligand binding to calmodulin has been studied extensively with (113)Cd NMR showing that the metal binding sites are not directly involved in the ligand binding. The (113)Cd chemical shifts are, however, exquisitely sensitive to minute changes in the metal ion environment.In the case of metallothionein, we will reflect upon how (113)Cd substitution and the establishment of specific Cd to Cys residue connectivity by proton-detected heteronuclear (1)H-(113)Cd multiple-quantum coherence methods (HMQC) was essential for the initial establishment of the 3D structure of metallothioneins, a protein family deficient in the regular secondary structural elements of α-helix and β-sheet and the first native protein identified with bound Cd. The (113)Cd NMR studies also enabled the characterization of the affinity of the individual sites for (113)Cd and, in competition experiments, for other divalent metal ions: Zn, Cu, and Hg.

Expression and function analysis of metallothionein in the testis of stone crab Charybdis japonica exposed to cadmium

Metallothionein (MT) participates in metallic homeostasis and detoxification in living animals. Previous studies have focused mainly on the functions of MT in vertebrates, but the functions of MT during spermiogenesis in invertebrates remain unclear. In order to investigate the functions of MT during spermiogenesis in the Japanese stone crab (Charybdis japonica), we identified the C. japonica MT complete cDNA sequence from the total RNA of the testis using RT-PCR and RACE. The 587 bp MT cDNA contains: an 80 bp 5' untranslated region, a 333 bp 3' untranslated region, and a 174 bp open reading frame. MT has 57 amino acids including 19 cysteines. The protein alignment between MT sequences of C. japonica and other crabs shows a high similarity and a strong identity in cysteine residues vital for the metal-binding affinity of MT. After the cadmium (Cd) exposure, the testis displays both abnormal morphology and MT mRNA expression both of which indicate a sensitive response of testis MT to Cd. Therefore, we suggest that MT is an excellent biomarker candidate for evaluating Cd pollution.

Solution structure of MSL2 CXC domain reveals an unusual Zn3Cys9 cluster and similarity to pre-SET domains of histone lysine methyltransferases

The dosage compensation complex (DCC) binds to single X chromosomes in Drosophila males and increases the transcription level of X-linked genes by approximately twofold. Male-specific lethal 2 (MSL2) together with MSL1 mediates the initial recruitment of the DCC to high-affinity sites in the X chromosome. MSL2 contains a DNA-binding cysteine-rich CXC domain that is important for X targeting. In this study, we determined the solution structure of MSL2 CXC domain by NMR spectroscopy. We identified three zinc ions in the CXC domain and determined the metal-to-cysteine connectivities from ¹H-¹¹³Cd correlation experiments. The structure reveals an unusual zinc-cysteine cluster composed of three zinc ions coordinated by six terminal and three bridging cysteines. The CXC domain exhibits unexpected structural homology to pre-SET motifs of histone lysine methyltransferases, expanding the distribution and structural diversity of the CXC domain superfamily. Our findings provide novel structural insight into the evolution and function of CXC domains.

A smallest 6 kda metalloprotease, mini-matrilysin, in living world: a revolutionary conserved zinc-dependent proteolytic domain- helix-loop-helix catalytic zinc binding domain (ZBD)

Background

The Aim of this study is to study the minimum zinc dependent metalloprotease catalytic folding motif, helix B Met loop-helix C, with proteolytic catalytic activities in metzincin super family. The metzincin super family share a catalytic domain consisting of a twisted five-stranded β sheet and three long α helices (A, B and C). The catalytic zinc is at the bottom of the cleft and is ligated by three His residues in the consensus sequence motif, HEXXHXXGXXH, which is located in helix B and part of the adjacent Met turn region. An interesting question is - what is the minimum portion of the enzyme that still possesses catalytic and inhibitor recognition?”

Methods

We have expressed a 60-residue truncated form of matrilysin which retains only the helix B-Met turn-helix C region and deletes helix A and the five-stranded β sheet which form the upper portion of the active cleft. This is only 1/4 of the full catalytic domain. The E. coli derived 6 kDa MMP-7 ZBD fragments were purified and refolded. The proteolytic activities were analyzed by Mca-Pro-Leu-Gly-Leu-Dpa-Ala-Arg-NH2 peptide assay and CM-transferrin zymography analysis. SC44463, BB94 and Phosphoramidon were computationally docked into the 3day structure of the human MMP7 ZBD and TAD and thermolysin using the docking program GOLD.

Results

This minimal 6 kDa matrilysin has been refolded and shown to have proteolytic activity in the Mca-Pro-Leu-Gly-Leu-Dpa-Ala-Arg-NH2 peptide assay. Triton X-100 and heparin are important factors in the refolding environment for this mini-enzyme matrilysin. This minienzyme has the proteolytic activity towards peptide substrate, but the hexamer and octamer of the mini MMP-7 complex demonstrates the CM-transferrin proteolytic activities in zymographic analysis. Peptide digestion is inhibited by SC44463, specific MMP7 inhibitors, but not phosphorimadon. Interestingly, the mini MMP-7 can be processed by autolysis and producing ~ 6 ~ 7 kDa fragments. Thus, many of the functions of the enzyme are retained indicating that the helix B-Met loop-helix C is the minimal functional “domain” found to date for the matrixin family.

Conclusions

The helix B-Met loop-helix C folding conserved in metalloprotease metzincin super family is able to facilitate proteolytic catalysis for specific substrate and inhibitor recognition. The autolysis processing and producing 6 kDa mini MMP-7 is the smallest metalloprotease in living world.

Isolation of metallothionein genes and in silico structural characterization of their proteins using molecular modeling from yak (Bos grunniens)

Yak metallothioneins (BgMTs) are cysteine-rich metal-chelating proteins with highly conserved cysteine residues in their amino acid sequences. The 3D structures of the Cd(7)-BgMTs reconstructed by molecular modeling included two domains: the β-domain with M(3)(S(cys))(9) metal-thiolate clusters and the α-domain with M(4)(S(cys))(11) metal-thiolate clusters. An unusual variant was found at position 30 (Cys30→Ser30) in BgMT-III, which is usually conserved in the mammalian MT-I/-II (Cys29) and MT-III (Cys30). The variant residue of BgMT-III may play a key role in yak genetic evolution, metal-binding activity, dynamic conformation, and heavy metal metabolism. BgMT-III contained a Thr insertion at position 5 (T(5)), which may loosen the structure of the β-domain of BgMT-III, and a conserved C(6)PCP(9) motif, which may provide an interacting surface for protein-protein interactions. There is also an acidic hexapeptide insertion (E(55)GAEAE(60)) that could regulate the particular interdomain interactions and lead to the conformational change in the β-domain.

Structural features specific to plant metallothioneins

The metallothionein (MT) superfamily combines a large variety of small cysteine-rich proteins from nearly all phyla of life that have the ability to coordinate various transition metal ions, including Zn(II), Cd(II), and Cu(I). The members of the plant MT family are characterized by great sequence diversity, requiring further subdivision into four subfamilies. Very peculiar and not well understood is the presence of rather long cysteine-free amino acid linkers between the cysteine-rich regions. In light of the distinct differences in sequence to MTs from other families, it seems obvious to assume that these differences will also be manifested on the structural level. This was already impressively demonstrated with the elucidation of the three-dimensional structure of the wheat E(c)-1 MT, which revealed two metal cluster arrangements previously unprecedented for any MT. However, as this structure is so far the only one available for the plant MT family, other sources of information are in high demand. In this review the focus is thus set on any structural features known, deduced, or assumed for the plant MT proteins. This includes the determination of secondary structural elements by circular dichroism, IR, and Raman spectroscopy, the analysis of the influence of the long linker regions, and the evaluation of the spatial arrangement of the sequence separated cysteine-rich regions with the aid of, e.g., limited proteolytic digestion. In addition, special attention is paid to the contents of divalent metal ions as the metal ion to cysteine ratios are important for predicting and understanding possible metal-thiolate cluster structures.

Metallothionein protein evolution: a miniassay

Metallothionein (MT) evolution is one of the most obscure yet fascinating aspects of the study of these atypical metal-binding peptides. The different members of the extremely heterogeneous MT protein superfamily probably evolved through a web of duplication, functional differentiation, and/or convergence events leading to the current scenario, which is particularly hard to interpret in terms of molecular evolution. Difficulties in drawing straight evolutionary relationships are reflected in the lack of definite MT classification criteria. Presently, MTs are categorized either according to a pure taxonomic clustering or depending on their metal binding preferences and specificities. Extremely well documented MT revisions were recently published. But beyond classic approaches, this review of MT protein evolution will bring together new aspects that have seldom been discussed before. Hence, the emergence of life on our planet, since metal ion utilization is accepted to be at the root of the emergence of living organisms, and global trends that underlie structural and functional MT diversification, will be presented. Major efforts are currently being devoted to identifying rules for function-constrained MT evolution that may be applied to different groups of organisms.

Protein and metal cluster structure of the wheat metallothionein domain γ-E(c)-1: the second part of the puzzle

Metallothioneins (MTs) are small cysteine-rich proteins coordinating various transition metal ions, including Zn(II), Cd(II), and Cu(I). MTs are ubiquitously present in all phyla, indicating a successful molecular concept for metal ion binding in all organisms. The plant MT E(c)-1 from Triticum aestivum, common bread wheat, is a Zn(II)-binding protein that comprises two domains and binds up to six metal ions. The structure of the C-terminal four metal ion binding β(E) domain was recently described. Here we present the structure of the N-terminal second domain, γ-E(c)-1, determined by NMR spectroscopy. The γ-E(c)-1 domain enfolds an M (2) (II) Cys(6) cluster and was characterized as part of the full-length Zn(6)E(c)-1 protein as well as in the form of the separately expressed domain, both in the Zn(II)-containing isoform and the Cd(II)-containing isoform. Extended X-ray absorption fine structure analysis of Zn(2)γ-E(c)-1 clearly shows the presence of a ZnS(4) coordination sphere with average Zn-S distances of 2.33 Å. (113)Cd NMR experiments were used to identify the M(II)-Cys connectivity pattern, and revealed two putative metal cluster conformations. In addition, the general metal ion coordination abilities of γ-E(c)-1 were probed with Cd(II) binding experiments as well as by pH titrations of the Zn(II) and Cd(II) forms, the latter suggesting an interaction of the γ domain and the β(E) domain within the full-length protein.

Metallothioneins: unparalleled diversity in structures and functions for metal ion homeostasis and more

Metallothioneins have been the subject of intense study for five decades, and have greatly inspired the development of bio-analytical methodologies including multi-dimensional and multi-nuclear NMR.With further advancements in molecular biology, protein science, and instrumental techniques, recent years have seen a renaissance of research into metallothioneins. The current report focuses on in vitro studies of so-called class II metallothioneins from a variety of phyla, highlighting the diversity of metallothioneins in terms of structure, biological functions, and molecular functions such as metal ion specificity, thermodynamic stabilities, and kinetic reactivity. We are still far from being able to predict any of these properties, and further efforts will be required to generate the knowledge that will enable a better understanding of what governs the biological and chemical properties of these unusual and intriguing small proteins.

Cadmium(II) complex formation with glutathione

Complex formation between heavy metal ions and glutathione (GSH) is considered as the initial step in many detoxification processes in living organisms. In this study the structure and coordination between the cadmium(II) ion and GSH were investigated in aqueous solutions (pH 7.5 and 11.0) and in the solid state, using a combination of spectroscopic techniques. The similarity of the Cd K-edge and L(3)-edge X-ray absorption spectra of the solid compound [Cd(GS)(GSH)]ClO(4).3H(2)O, precipitating at pH 3.0, with the previously studied cysteine compound {Cd(HCys)(2).H(2)O}(2).H(3)O(+).ClO(4) (-) corresponds to Cd(S-GS)(3)O (dominating) and Cd(S-GS)(4) four-coordination within oligomeric complexes with mean bond distances of 2.51 +/- 0.02 A for Cd-S and 2.24 +/- 0.04 A for Cd-O. For cadmium(II) solutions (C (Cd(II)) approximately 0.05 M) at pH 7.5 with moderate excess of GSH (C (GSH)/C (Cd(II)) = 3.0-5.0), a mix of Cd(S-GS)(3)O (dominating) and Cd(S-GS)(4) species is consistent with the broad (113)Cd NMR resonances in the range 632-658 ppm. In alkaline solutions (pH 11.0 and C (GSH)/C (Cd(II)) = 2.0 or 3.0), two distinct peaks at 322 and 674 ppm are obtained. The first peak indicates six-coordinated mononuclear and dinuclear complexes with CdS(2)N(2)(N/O)(2) and CdSN(3)O(2) coordination in fast exchange, whereas the second corresponds to Cd(S-GS)(4) sites. At high ligand excess the tetrathiolate complex, Cd(S-GS)(4), characterized by a sharp delta((113)Cd) NMR signal at 677 ppm, predominates. The average Cd-S distance, obtained from the X-ray absorption spectra, varied within a narrow range, 2.49-2.53 A, for all solutions (pH 7.5 and 11.0) regardless of the coordination geometry.

Kinetic analysis of arsenic-metalation of human metallothionein: significance of the two-domain structure

Metallothionein (MT) is ubiquitous in Nature, underlying MT's importance in the cellular chemistry of metals. Mammalian MT consists of two metal-binding domains while microorganisms like cyanobacteria consist of a single metal-binding domain MT. The evolution of a two-domain protein has been speculated on for some time; however, no conclusive evidence explaining the evolutionary necessity of the two-domain structure has been reported. The results presented in this report provide the complete kinetic analysis and subsequent mechanism of the As(3+)-metalation of the two-domain beta alpha hMT and the isolated single domain fragments using time- and temperature-resolved electrospray ionization mass spectrometry. The mechanism for beta alpha hMT binding As(3+) is noncooperative and involves six sequential bimolecular reactions in which the alpha domain binds As(3+) first followed by the beta domain. At room temperature (295 K) and pH 3.5, the sequential individual rate constants, k(n) (n = 1-6) for the As(3+)-metalation of beta alpha hMT starting at k(1beta alpha) are 25, 24, 19, 14, 8.7, and 3.7 M(-1)s(-1). The six rate constants follow an almost linear trend directly dependent on the number of unoccupied sites for the incoming metal. Analysis of the temperature-dependent kinetic electrospray ionization mass spectra data allowed determination of the activation energy for the formation of As(1)-H(17)-beta alpha hMT (14 kJ mol(-1)) and As(2-6)-beta alpha hMT (22 kJ mol(-1)). On the basis of the increased rate of metalation for the two-domain protein when compared with the isolated single-domain, we propose that there is an evolutionary advantage for the two-domain MT structures in higher organism, which allows MT to bind metals faster and, therefore, be a more efficient metal scavenger.

Metallothioneins in Aquatic Organisms: Fish, Crustaceans, Molluscs, and Echinoderms

Metallothioneins (MTs) have been described in a wide range of organisms, from bacteria to mammals, thus representing an interesting example of evolutionary molecular adaptation. If the moderate variability of MTs across phylogenetically distant organisms reflects their highly conserved function, the specific environmental requirements may explain the multiplicity of isoforms also in the same organism. The MT polymorphism is particularly important in invertebrates with respect to vertebrates. This review is an attempt to summarize the knowledge about MTs from aquatic animals, both vertebrates and invertebrates, to gain new insights into the structure-function relationship of this class of proteins. The large and increasing literature on MTs indicates that MTs from aquatic vertebrates are rather similar to mammalian counterparts, whereas a variety of structures have been described in invertebrates. Although the prototypical αβ-domain organization of vertebrate MTs has been observed in most invertebrate isoforms, some invertebrate MTs display alternative structures in which the canonical organization has been modified, such as the ββ-domain, the αββ-domain, and the multiple α-domain structures of oyster MTs, and the inverted βα-domain organization of sea urchin MTs. In this review we emphasize three major taxa of aquatic invertebrates, the molluscs, the crustaceans and the echinoderms, although some data have been reported for other invertebrates.

The metal-binding features of the recombinant mussel Mytilus edulis MT-10-IV metallothionein

In contrast with the paradigmatic mammalian metallothioneins (MTs), mollusc MT systems consist at least of a high-cadmium induced form, possibly involved in detoxification, and another isoform either constitutive or regulated by essential metals and probably associated with housekeeping metabolism. With the aim of providing a deeper characterization of the coordination features of a molluscan MT peptide of the latter kind, we have analyzed here the metal-binding abilities of the recombinant MeMT-10-IV isoform of Mytilus edulis (MeMT). Also, comparison with other MTs of this type has been undertaken. A synthetic complementary DNA was constructed, cloned and expressed into two Escherichia coli systems. Upon zinc coordination, MeMT folds in vivo into highly chiral and stable Zn(7) complexes, with an exceptional reluctance to fully substitute cadmium(II) and/or copper(I) for zinc(II). In vivo cadmium binding leads to homometallic Cd(7) complexes that structurally differ from any of the in vitro prepared Cd(7) complexes. Homometallic Cu-MeMT can only be obtained in vitro from Zn(7)-MeMT after a great molar excess of copper(I) has been added. In vivo, two different heterometallic Zn,Cu-MeMT complexes are recovered, which nicely correspond to two distinct stages of the in vitro zinc/copper replacement. These MeMT metal-binding features are consistent with a physiological role related to basal/housekeeping metal, mainly zinc, metabolism, and confirm the correspondence between the MeMT gene response pattern and the functional properties of the encoded protein.

Zinc, cadmium, and mercury 1,2-benzenedithiolates with intramolecular NH...S hydrogen bonds

Mononuclear Zn, Cd, and Hg 1,2-benzenedithiolates with intramolecular NH...S hydrogen bonds, [M(II){1,2-S2-3,6-(RCONH)2C6H2}2](2-) (R = CH 3, t-Bu; M = Zn, Cd, Hg), were synthesized and characterized by X-ray analysis and spectral measurements. The presence of intramolecular NH...S hydrogen bonds was established by the IR spectra. (199)Hg and (113)Cd nuclear magnetic resonance showed a stabilized four-thiolate coordinated structure and suggested the influence of the NH...S hydrogen bonds to ppi(Hg)-ppi(S) interactions. The NH stretching bands show that the NH...S hydrogen bonds in Cd and Hg complexes are stronger than those in the corresponding Zn complex. These results are supported by theoretical calculations. The experimental and theoretical results suggested that the NH...S hydrogen bond influences the efficient capture of toxic Cd and Hg ions by metallothioneins.

Metal ion binding properties of Tricium aestivum Ec-1 metallothionein: evidence supporting two separate metal thiolate clusters

Metallothioneins are ubiquitous low molecular mass, cysteine-rich proteins with an extraordinary high metal ion content. In contrast to the situation for the vertebrate forms, information regarding the properties of members of the plant metallothionein family is still scarce. We present the first spectroscopic investigation aiming to elucidate the metal ion binding properties and metal thiolate cluster formation of the Triticum [corrected] aestivum (common wheat) early cysteine-labeled plant metallothionein (Ec-1). For this, the protein was overexpressed recombinantly in Escherichia coli. Recombinant Ec-1 is able to bind a total of six divalent d10 metal ions in a metal thiolate cluster arrangement. The pH stability of the zinc and cadmium clusters investigated is comparable to stabilities found for mammalian metallothioneins. Using cobalt(II) as a paramagnetic probe, we were able to show the onset of cluster formation taking place with the addition of a fourth metal ion equivalent to the apo protein. Limited proteolytic digestion experiments complemented with mass spectrometry and amino acid analysis provide clear evidence for the presence of two separate metal thiolate clusters. One cluster consists of four metal ions and is made up by a part of the protein containing 11 cysteine residues, comparable to the situation found in the mammalian counterparts. The second cluster features two metal ions coordinated by six cysteine residues. The occurrence of the latter cluster is unprecedented in the metallothionein superfamily so far.

Structural study of the zinc and cadmium complexes of a type 2 plant (Quercus suber) metallothionein: Insights by vibrational spectroscopy

Zn- and Cd-complexes of Quercus suber metallothionein (QsMT) were obtained by in vivo-synthesis, in order to obtain physiologically representative aggregates, and characterized by spectrometric and spectroscopic methods. The secondary structure elements and the coordination environments of the metal binding sites of the two aggregates were determined, as well as the main metal-containing species formed. The results obtained from the analysis of the Raman and IR spectra reveal that these metal-MT complexes predominantly contain beta-sheet elements (about 60%), whereas they lack alpha-helices. These structural features slightly depend on the divalent metal bound. In particular, Cd(II) binding to QsMT induces a slight increase of the beta-sheet percentage, as well as a decrease in beta-turn elements with respect to Zn(II) binding. Conversely, the in vivo capability of QsMT to inglobe metal and sulfide ions is metal-depending. Spectroscopic vibrational data also confirm the presence of sulfide ligands in the metal clusters of both Zn- and Cd-QsMT, while the participation of the spacer His residue in metal coordination was only found in Cd-QsMT, in agreement with the CD results. Overall data suggest different coordination environments for Zn(II) and Cd(II) ions in QsMT.

Cloning, expression and characterization of metallothionein from the Antarctic clam Laternula elliptica

The genes for two apparent subtypes of metallothionein (MT) isoform were isolated from the Antarctic clam Laternula elliptica. Determination of the nucleotide sequence showed that the gene consists of 222 bp that code a 73-amino acid protein. The comparison between MT cDNA sequences of L. elliptica and other bivalves showed strong homologies on positions of cysteine residues, which are important for their metal binding abilities. The gene for the MT was inserted into a pET vector and overexpressed as a carboxyl terminal extension of glutathionein-S-transferase (GST) in Escherichia coli. After the GST fusion proteins had been purified by glutathione-Sepharose affinity chromatography column and digested with enterokinase, the MT was purified with gel filtration and analyzed for its biochemical properties. Recombinant MTs were reconstituted with Cd, Cu, and Zn, and kinetic studies of the reactions with electrophilic disulphide, DTNB, were investigated to explore their metal binding ability. It is revealed that the Cd-MT and Zn-MT react with DTNB biphasically, and that Zn-MT reacts with DTNB more rapidly, and with a significantly greater pseudo-first-order rate constant. Cu-MT reacts monophasically and releases metal slowly from MT.